1. Field of the Invention
The present invention relates to a mask used in a photolithographic system in fabrication of semiconductor devices and the like.
2. Description of the Prior Arts
ULSI semiconductor devices which have been produced in recent years assemble a large number of transistors and wirings in micron photolithography on a single silicon substrate. Used for formation of the patterns of the transistors and wirings in submicrons is a photolithographic technology, such as a stepper (step and repeat photolithographic system with demagnification), transferring a mask's pattern in a reduced size (usually to one fifth) onto a photosensitive resin (resist) layer formed on the silicon substrate of the semiconductor devices.
The photolithographic system (the stepper) which is now applied to mass production of 1 MB and 4 MB DRAM devices uses the mask patterns having the respective minimum line widths of 1.2 and 0.8 .mu.m for each DRAM device, and generally adopts emission lines called g-light of 436 nm emitted by an ultra high-pressure mercury-vapor lamp, while emission lines called i-light of 365 nm emitted by the same lamp has been locally used.
It is expected that each of 16 MB and 64 MB DRAM devices intended to be produced in the future will use 0.6 to 0.5 .mu.m and 0.4 to 0.3 .mu.m of the minimum line widths, respectively. For mass production of these semiconductor devices, an advanced photolithographic technology with the higher resolution needs to be developed. In order to improve the resolution by using light of shorter wavelengths, use of the i-lines in place of the g-lines, and KrF excimer laser of 248 nm have been studied.
In the above photolithographic system using the mask comprising a transparent substrate made of quartz with an opaque pattern generally of a thin metal layer disposed thereon, light is applied to the mask from the back thereof and light flux passing through the mask is converged on the substrate to form an image in a reduced size through a projection lens. Contrast of the image on the substrate is deteriorated due to diffraction of the light at the mask pattern edge as the mask pattern size approaches specific wave-lengths of the light, resulting in the extremely poor resolution of the light in comparison with critical resolution defined by NA of numerical aperture of the projection lens and the wavelengths of the light. A new method to improve a pattern structure on the mask, achieve a higher contrast of the image and remarkably improve the resolution of the photolithography has been proposed previously.
The above previously proposed method uses a mask called a phase shifting mask as exemplified in FIG. 5 which mask comprises a transparent quartz substrate 51 to admit light of any wavelengths, a chromium thin film 52 for obstructing applied light, and a transparent thin film 53 (phase shifting film) to admit applied light, the thickness t.sub.s of the thin film 53 having the following relationship with its refractive index n and wavelengths .lambda. of applied light: EQU T.sub.s =.lambda./{2.multidot.(n-1)} (1)
which condition sets the phase of applied light passing through the thin film 53 to be shifted half the cycle of waves. The phase shifting mask also has an opening 54 provided as conventionally and fine apertures 55 adjacent to the opening 54. The fine apertures 55, 55' do not dissect image by themselves and are disposed with the phase shifting film 53.
Light from the opening 54 is shifted by 180.degree. relative to light from the apertures 55, so that light wave diffracted from the opening 54 to its adjacent area is offset with light wave from the apertures 55. Accordingly, light on the projection area is restrained from effusing from the opening 54 to its adjacent area, thereby improving contrast of the projected image.
Hence, the phase shifting mask improves contrast of image projected on the resist layer to have the practical resolution largely improved. For preparation of the phase shifting mask, a first resist image for a chromium pattern is first drawn by use of electron beam lithographic system, and a second resist image is then drawn for defining a phase shifting film. As a result, the system has to conduct twice the electron beam drawing which takes longer time and also to align the first pattern with the second pattern with high accuracy, leading to a complex and expensive technology for preparation of mask.
To solve the problems, another phase shifting mask shown in FIG. 6 has been proposed, which comprises a transparent quartz substrate 61 to admit applied light of any wavelengths, a chromium film 62 obstructing the applied light and a phase shifting film 63 whose thickness satisfies the relationship of the foregoing formula (1). For preparation of the phase shifting mask, an opening 64 is first formed in a resist film (not shown) by use of electron beam lithographic system and the chromium film 62 is then etched and removed at the part of the opening 64, while the resist film is also removed. Next, a resist film for forming the phase shifting film 63 is formed and ultraviolet is applied from the back of the substrate to the resist film at the opening 64 which resist film is then developed and removed. The chromium film 62 is etched partially under the resist film to have an opening larger than the opening 64 and having a region 65 over which the resist film hangs. Light passing through the region 65 and that passing through the opening 64 are different in phase at 180.degree. from each other, so that both light is offset with each other when they are overlapped in a projected image, thereby preventing the light from effusing from the region 64 to its adjacent part. Hence, dispersion of photo-intensity on an image formation surface is sharp to improve the practical resolution of photolithography.
The preparation method for this phase shifting mask does not need to increase times of drawing by electron beam lithographic system, in turn, the number of steps for preparation of the mask. However, since the resist film kept for forming the phase shifting film is fragile, it is hard to wash and remove foreign particles on the mask. Thus, the method is not practical.
The phase shifting masks as discussed above or related masks are disclosed in e.g.,
Japanese Patent Publication No. 62(1987)-50811, Japanese Unexamined Patent Publication Nos. 58(1983)-173744, 62(1987)-67514, 1(1990)-147457 & 1(1990)-283925 and IEDM 1989, p57 (New Phase Shifting Mask with Self-aligned Phase Shifters for a Quarter Micron Photolithography).
As seen from the above, the phase shifting mask previously proposed has the above problems in practical use, and any phase shifting masks simple in fabrication and having sufficient durability have been desired.